This invention relates to a water addition process for increasing the activity of a Fischer-Tropsch, fixed bed or slurry bed, hydrocarbon synthesis reaction over a catalyst comprising metals selected from ruthenium, cobalt and mixtures thereof. The catalyst preferably will be supported on a support containing titania. However, where cobalt metal is used, the catalyst may be in bulk form. More particularly, this invention relates to increasing catalyst activity by decreasing the methane make, increasing the CO conversion and increasing C.sub.5 + hydrocarbon selectivity of a Fischer-Tropsch hydrocarbon synthesis process which comprises feeding a mixture of H.sub.2, CO and H.sub.2 O into a reaction zone containing a catalyst comprising metals selected from ruthenium, cobalt and mixtures thereof in the reduced, metallic form wherein said mixture contacts said catalyst at elevated temperature for a time sufficient to convert at least a portion of said feed to C.sub.5 + hydrocarbons.